This invention relates generally to radio frequency antenna and more particularly to radio frequency antenna adapted to operate with circular polarization.
As is known in the art, it is frequently desirable to provide a radio frequency antenna adapted to operate with circular polarization. One such antenna is described in U.S. Pat. No. 4,353,072, entitled "Circularly Polarized Radio Frequency Antenna", issued Oct. 5, 1982, inventor George J. Monser and assigned to the same assignee as the present invention. As described therein, the radio frequency antenna includes an antenna element having a forward, open-ended waveguide section, the upper and lower relatively wide side walls thereof being provided by the ground planes of a pair of microwave circuits. A first feed is disposed adjacent the rearward end of the waveguide section for radiating radio frequency energy having an electric field normal to the upper and lower wide walls of the waveguide section (i.e., a vertical polarization having a phase center at the center of the open-ended waveguide). Each one of the pair of microwave circuits includes a strip transmission line circuit having disposed adjacent the forward, open-ended waveguide section an array of radiating elements and a rearward feed for radiating from the array of radiating elements radio frequency energy having an electric field parallel to the plane of the upper and lower side walls (i.e., a horizontal polarization). Thus, by feeding equal, in-phase signals to the pair of microwave circuits and by feeding a 90.degree. phase shifted signal to the first feed, circularly polarized energy is radiated by the antenna element. Further, with such arrangement, the resultant phase center of the horizontally polarized energy is disposed at the center of the open-ended waveguide and, therefore, both the horizontal and vertical polarization components have coincident phase centers. While such antenna element does provide circular polarization, when used in relatively large arrays, a relatively large power distribution network is required to feed equal signals to each of the pair of microwave circuits of each of the antenna elements in the array. Further, such network may add loss and unwanted phase shift which with increasing scan angles, leads to non-coincident phase center and reduced quality of the circular polarization.